Method and apparatus for determining time from target by signal reflection



Dec. 12, 1950 Filed NOV. 30, 1946 E. O. KEIZER METHOD AND APPARATUS FOR DETERMINING TIME FROM TARGET BY SIGNAL REFLECTION 3 Sheets-Sheet 2 AT TURNEY 10 11 Egg. 5

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H E c INI 'ENTQR. '2 Eugene OKezzer 3 I BY i A 7" TORNEY Patented Dec. 12, 1950 METHOD AND APPARATUS FOR DETERMIIN- ING TIME REFLECTION FROM TARGET BY SIGNAL Eugene 0. Keizer, Princeton, N. J., assignor to Radio Corporation of America, a corporation of 7 Delaware Application November 30, 1946, Serial No. 713,310

11 Claims.

My invention relates to radio apparatus for utilizing signal reflected from a target to detergmine the time required to reach the target and to a method of determining said time.

-An object of the invention is to provide an improved means for and method of determining time from target.

. r A further object of the invention is to provide an improved means for and method of releasing a bomb or the like at the correct time for hitting i :a target. L :According to one embodiment of the present invention an unmodulated radio carrier wave is radiated continuously toward a target and the reflected signal is mixed in a detector with carrier wave signal obtained directly from the transjrnitter whereby an audio beat frequency due to :the Doppler effect is obtained. For the purpose of illustration it may be assumed that the apparatus comprising the transmitter, detector and other apparatus described hereinafter are located in an airplane that is approaching the target which may be a ship that is to be bombed.

Instead of relying on the Doppler effect to provide an audio frequency signal, it may be pre- 1 ferred to modulate the carrier wave with a continuous audio frequency signal such as a signal having a frequency of 3000 cycles per second. In this case, the modulating signal should have a irequencydifiering from the Doppler frequency by an amount sufiicient to permit separation of the two audio frequencies by suitable filtering.

, The audio frequency signal is supplied to apparatus that provides a signal output that is inversely proportional to the time required for the airplane to reach the target, hereinafter referred to as time to target and represented by T.

The invention is based upon the fact that as the target is approached the ratio of the time rate of change of signal strength to signal strength is inversely proportional to T and is independent 1 v will now be described with reference to the acof the size of the target and is independent of the speed of approach to the target. The speed of approach, however, should be constant.

That the said ratio is proportional to signal at the airplane carrying theapp'aratus, K is a constant depending upon the radio transmitter and the target, and D is the distance fro the airplaneto the target. Then 3 dE- 2K rate of change of signal strength versus Eff 3 distance dD -2K D '2 proportionate rate of change of i "K' signal strength versus distance a 'z? s a where S is the speed of approach to the target. Then multiplying above equation by S gig dD -2S T' -T in dD dt 2S E D dt S -s )ercen t change of voltage in a giventnne If speedis constant, D=ST and dt 2S 2 E ST T Thus it hasbeen shown that the per cent change of voltage in a given time equals a constant time 1/ T. Apparatus for obtaining an outputequal to s di companying drawing in which: u. Figure 1- is a block diagram illustrating one embodimentof the invention,

? embodiment of the invention,

Figures 4 and 4a are circuit and block diagrams, respectively, illustrating embodiments of the in 'vention that may be substituted for the apparatus shown in Fig. 2, 1

Figure 5=is a block diagram of apparatus that maybe employed with apparatus such as shown resistor 21.

in Fig. 1 for improving the operation in the event of signal fading,

Figure 6 is a circuit diagram of the apparatus shown in Fig. 5, and

Figure 7 is a graph that is referred to in explaining the operation of the apparatus shown in Figs. and 6.

In the several figures, similar parts are indicated by similar reference characters.

Referring to Fig. 1, there" is shown a radio transmitter it that radiates continuously an unmodulated carrier Wave from a suitable antenna II. This wave is reflected from the target or other reflecting object and is picked up by an antenna 12 and supplied to a detector i3 where it beats with signal supplied over leads It from the transmitter it. The resulting beat frequency signal is amplified by an audio frequency ame plifier l6.

Referring to Fig. 1 and Fig. 2, signal from the amplifier i6 is supplied to a detector I! having a slow time constant and also to a detector l8 having a comparatively fast time constant. Automatic gain control or A. V. 0. signal is applied from the slow detector IT to the audio frequency amplifier l6. ,The outputs of detectors l1 and l a are supplied to an output balance circuit l9 matically as As shown in Fig. 2, the detectors I1 and I8 comprise diodes Ila and l8a that are oppositely connected so that their outputs are of opposite polarity. Signal is applied from amplifier it through coupling capacitors 2| and 22 to the diodes Ila, and lBa, respectively. Delay voltage for the detectors may be supplied from a voltage source 23 that has resistors 24 and 26 connected thereacross, the voltage applied across resistors 24 and 26 being adjustable by means of a variable The voltage applied to the diodes is further adjustable bymeans of variable resistor 26.

The junction point of resistors 24 and 26 is grounded. Delay voltage is applied from the ungrounded end of resistor'26; to the cathode of diode Ila through a filter comprising a resistor 28 and a capacitor 29. Similarly, delay voltage is applied from the ungrounded end of the resistor "24 to the anode of diode l8a through a filter comprising a resistor 3| and a capacitor 32.

The diode Ha has an output resistor 33 connected between its anode and ground while the diode i8a has an output resistor 34 connected between its cathode and ground. Resistors 33 and 34 are shunted by capacitors 36 and 31, re-

spectively. r

The negative polarity output signal of the diode [1a is applied to one end of a resistor 39 of the I balance circuit I!) through a comparatively. slow time constant filter comprising a resistor 4| and :acapacitor 42 and through a resistor 43. The

positive polarity output signal of the diode lBa is applied to the other end of the balance. resistor 39 through a comparatively fast time constant filter comprising a resistor 44 and a capacitor 48 and through a resistor 41. An adjustable tap 48 on the balance resistor 39is connected to the grid of a vacuum tube 49 through a filter comprising a resistor 5! and a capacitor 62. A

4. relay 50 is actuated when the voltage at tap lfi reaches a predetermined value, that is, when reaches said predetermined value.

The tap 48 is adjusted to a point of zero voltage with respect to ground for the condition of equal voltage output from the diodes Ila and 18a. For the purpose of this adjustment, an unchanging signal is applied to the audio amplifier. In Fig. 2, circuit values are indicated in megohms and microfarads merely by way of example.

When the distance between the radio equipment and the target is changing, there will be a voltage at the tap 48 equal to dill because of the A. V. C. connection 53 from the slow time constant filter 4|, 42 to the audio amplifier [6. If, for example, an airplane carrying the equipment is approaching a target, the signal supplied to the amplifier IE will be increasing in amplitude. Although the A. V. C. will tend to hold the output of the amplifier H5 at a constant value this A. V. C. control will lag the increase in the strength of input signal at the amplifier [6. Since there is less time lag in the fast detector [8 than in the slow detector II, the voltage applied from detector [8 to the balance resistor 39 will be of greater amplitude than the voltage applied from detector H to the resistor 39, and the more rapid the increase in signal strength, the greaterwill be the difference in the two voltages. Therefore, a positive voltage appears at tap 48 and, as will now be shown, this voltage equals Assuming a good or substantially flat A. V. C. and the use of a delay voltage:

Yj=instantaneous audio output.

Vi=Vi averaged over the time constant of the slow detector.

Also, l7ri=K for steady input voltage to the audio amplifier or for very slow changes in input, such as very slow fading, and for changes of the order of the time constant of the slow detector.

E=received signal strength substantially averaged over the period of the time constant of the slow detector.

E=instantaneous received signal strength.

G=gain of audio amplifier.

sue

'5 Total output=fast detector output minus slow detector output, or

9 "Since p 100=percentage change of input voltage in a given time, it follows that l dt II :bpK K of first mathematical development.

It is apparent, referring to the equation rig dt 2 F T that the voltage output is inversely proportional to the time required to reach the target.

As shown in Figure 3, the invention may be applied to a superheterodyne type of system, a system of this general type beLng described in Chaifee Re. 21,955, November 25, 1941. In this type of system an oscillator 6| operating at the desired intermediate frequency supplies signal to a mixer 62 where it is mixed with signal from the transmitter iii. The resulting signal, which includes upper and lower side bands, is supplied to a side-band filter 63 which supplies one of the side-band signals only to the detector l3. The side-band signal and the received signal mix in the detector It to produce a signal having the desired intermediate frequency, and this signal is amplified by an I.F. amplifier 64.

,The amplified I.-F. signal is supplied toa sec- ,ond detector 56 where it is mixed with signal from the LE. oscillator 61 to produce the beat frequency signal. This method of detecting the beat frequency signal in a system of this type is described and claimed in application Serial No. 507,221, filed October 20, 1943, now Patent No. 2,466,532, issued April 5, 1949, in the name of W. L. Carlson. The signal from detector 66 is supplied to the audio frequency amplifier l6.

In this system as shown in Figure 3, the A. V. C. signal from the slow detector I1 (Figure 1) is applied to the I.-F. amplifier 64 to control its gain. The operation for determining time from target is the same as previously described, the apparatus which follows the audio amplifier l6 being the same as shown in Figure 2, for example.

It should be understood that while the system of the present invention may be installed on aircraft and used for automatically releasing bombs at the proper time to hit a target, for example, the use of the invention is not so limited. For instance, the apparatus maybe installed on the ground and the radio wave directed toward an approaching aircraft or. missile from which the wave is reflected. This may be useful in directing the blind landing of an aircraft, for example. Also, the apparatus may be installed in a guided missile to provide time from target in formation for its control.

Figure 4 shows a simplified circuit that may be employed in place of the one shown in Figure 2. In this'circuit no A. V. C. is employed for the audio-amplifier Hi. The output of amplifier I6 is applied through a capacitor 6'! to the anode 68 of a diode detector tube 6%. A leak resistor H is connected between the anode 68 and ground, and an output resistor 12 is connected between the diode cathode l3 and ground.

The detector 69 may be a peak voltage detector, for example, although it need not be of this type. However, the time constant of the detector must be longenough for good detector action but short enough so that the detector output follows 'the signal strength change due to the changing distance between the aircraft and the target.

The detector output of voltage E is applied through resistors 74 and F6 to the anode l1 and the grid 18, respectively, of a triode 19 which in the example is of the 6J5 type. The cathode 8| of tri0de'19 is grounded.

The signal voltage 1) at the anode Ti is proportional to the logarithm of the total applied signal. This result can be obtained by a method of cut and try using difierent circuit values and different type tubes. The circuit example illustrated was found satisfactory. It will be noted that the resistor M and the cathode-anode impedance of the tube 19 are in series with each other to form a potentiometer connected across the diode output resistor 13. It will also be noted that as the positive signal voltage E increases in value, the said cathode-anode impedance is decreased. It is the voltage appearing across said cathode-anode impedance, i. e., the voltage 1; that is applied through a differentiating circuit and filter to the relay tube 49.

The difierentiatingcircuit comprises a capacitor 82 and a resistor 83 connected in series with each other and between the anode 1! and ground. The resulting signal appearing across resistor 83 is dB 2 dE and therefore is a constant times Therefore,

rig an an all dt E dt E Therefore, the output of the differentiating circuit 82, 83 is the'voltage that has been shown to be equal to a constant times the reciprocal of the time T from target.

In Figure 4 certain circuit values are given in ohms, megohms and microfarads merely by way of example,

The invention is not limited to the use of the circuit I4, 16, T9 for obtaining the log of the voltage E. For example, the value v=log E may be obtained by employing an ordinary amplifier provided with a fast-acting automatic volume control circuit and withoutany zero delay volt- Figures 5 and 6 show in block and circuit diagram, respectively, apparatus for minimizing the effect of. signal fluctuations or fading. In.- stead of the signal being applied directly to the grid of the relay tube 49 as in Figures 2 and 4, it is applied by way of an amplifier tube and relay unit A and a. low pass filter B to a comparator tube and motor circuit C which in turn. controls a smoother potentiometer D that supplies the voltage corresponding to to the grid of relay tube 49 of the release amplifier and relay unit R.

Referring more specifically to the circuit diagram of Figure 6, the unit A comprises an amplifier tube 9! having the coil of a relay 92 connected in its anode circuit. The tube 9i and relay 92 are adjusted so that the armature of relay 92 pulls up to close the circuit of a motor 93 in the unit C at some time before the release relay unit R is to be actuated.

The unit C includes, in addition to the motor 93, an amplifier tube 94 which has its cathodeanode impedance in series with the motor 93. When the armature of relay 92 is pulled up, the motor circuit is closed. This circuit may be traced from the +3 terminal through the relay armature, the motor 93, the tube 94, and through a cathode resistor 96 to ground.

The filter 3 comprises a-seriesresistors-l and a shunt capacitor 98. The grid and cathode of the tube 94 are connected to opposite sides of the capacitor 98 whereby the direct-current change on capacitor 98 tends to make the grid of tube 94- more positive as the signal from the unit it increases in amplitude.

The smoother potentiometer D comprises a resistor NH and a resistor P connected between +13 and ground.v The resistor P has a variable f.

tap H32 which is mechanically coupled. to. the motor 93. The. potentiometer resistor P is wound to have the characteristic shown in, Figure 7.

The tap I92 is connected electrically to the grid The circuit operation is asiollowsi H Assume that an aircraft carrying the apparatus of the invention is approaching a target, that the relay 92 has been closed and that there is no iading or fluctuation of the received signal. Then the grid of the tube 94 has an increasingly positive voltage applied to it through the filter resistor 91, and, also, the cathode of the tube 94 has an increasingly positive voltage applied to it since the motor is running and driving the tap )2 toward the top of the potentiometer resistor P. The circuit adjustment and the resistance winding of resistor P are such that, for the assumedv condition, the grid and cathode of tube 94 increase positively by the sameamount whereby thereis no voltage difference appearing across the capacitor 98.

Assume now that the signal fades away for a period less than the one second time constant of the filter B. Then the motor 93 will continue to run and cause the voltage I02 to become increasingly positive. Therefore, the release relay R will be operated at substantially the correct time even if the signal fades away just before the correct release time. There will be a slight error in this case because the cathode of the tube 94 smoothing circuit.

Where the received signal is fluctuating, its

mean or average value may be taken as the con runs faster.

;the. fluctuating signal itself.

rect value, and the one at which the voltages on the grid and cathode of the tube 94 would change like amounts. Assume the signal voltage is above this average value for a period of a half second, for example. Then the grid of tube 96 becomes positive with respect to the cathode, the impedance of the tube 94 is decreased, and the motor 93 On the negative half cycle of the signal fluctuation, on. the other hand, the motor 93 slows down some. The result is an averaging action which results in the potential at the tap W2 always being at much more nearly the correct value than is the instantaneous potential of I Consequently, the errors inthe time of actuation of the release relay are substantially reduced.

In Figure 6 certain circuit values are indicated in ohms and microiarads merely by way of ,ex-

= ample.

I claim as my invention: 1. The method of determining the time required for two relatively moving objects to meet where their speed of approach is constant which of a cathode follower tube H13 that has the resistor 96 in its cathode circuit. The cathode of the tube 94 is connected to the cathodeofi the tube W3 whereby the cathode bias. of tube 94 with respect to ground is determined by the position oi the tap W2. It will be. apparent thatv the impedance of the tube 94, and, there-fare the current through the motor 93, is controlled both by the position of the tap I02 and the amplitude of the output of the filter B.

The tap W2 is also electrically connected to the grid of therelay amplifier tube 49, preferably through a filter resistor I04 which, with a filter capacitor 106, reduces any noisev voltage fluctuations at said grid.

,jwhereby, the value comprises continuously radiating a radio wave from one of said objects toward the other object whereby said wave is reflected back to said one object with a field strength E at said. one object, and deriving, at said one object the value n a: E

is determined where T is the time required for prises means for continuously radiating a radio wave from one of said objects toward the other object whereby said wave is reflected back to said one object with a field strength E at said one object, and means for determining at said one object the value whereby the value is determined where T is the time required for said objects to meet.

3. The method of determining the time required for an object such as an aircraft or missile to reach a target which comprises the steps of moving said object toward said target at a constant speed, continuously radiating a radio wave toward saidtarget whereby it is reflected back to said object with a field strength E at said object, and deriving at said object the value ig dt E whereby the value is determined where T is the time required for the object to reach the target.

4. Apparatus for installation in an object such as an aircraft or missile for determining the time required for said object to reach a target that it is approaching at constant speed, said apparatus comprising means for continuously radiating a radio Wave toward said target whereby said wave is reflected back to said object with a field strength E at said object, means for receiving said reflected wave, and means for producing from said received wave a voltage having the value (112 dt E whereby the value is determined where T is the time required for said object to reach said target.

5. The invention according to claim 2 wherein said means for determining the value comprises an amplifier, a detector having a comparatively slow time constant, a detector having a comparatively fast time constant, means for supplying the output of said amplifier to each of said detectors, an automatic gain control circuit for said amplifier, said gain control circuit comprising a connection from the output circuit of said slow-time-constant detector to said amplifier, and means for connecting the output circuits of said two detectors in voltage opposition whereby the resulting output is a voltage equal to g dt 10 6. The invention according to claim 2 wherein said means for determining the value comprises means for producing a voltage '0 when the voltage E is applied to said means where v is defined by the equation v=1og E, and further comprising means for difierentiating the voltage 1) to obtain the voltage defined by the equation m dt E dt 7. Apparatus to which a signal having a voltage E is to be applied, said apparatus comprising an amplifier followed by a detector, a fastacting automatic volume control circuit for said amplifier, said control circuit including a con nection from said detector to said amplifier, said controlled amplifier having the characteristic that the voltage output 2) of said detector is given by the equation v=log E, and means for diflerentiating the voltage 1) to obtain the voltage defined by the equation an dt E dt 8. The invention according to claim 2 wherein said means for determining the value gg dt comprises an amplifier followed by a detector, 21' fast-acting automatic volume control circuit for said amplifier, said control circuit including a connection from said detector to said amplifier, said controlled amplifier having the characteristic that the voltage output 12 of said detector is given by the equation v=log E, and means for diiferentiating the voltage 1: to obtain the voltage defined by the equation an (it E dt 9. Apparatus to which a signal having a volttage E is to be applied, said apparatus comprising a triode having a cathode, a control grid and an anode, said anode being connected through a resistor to an input terminal, said control grid being connected through a second resistor to said input terminal, means for applying said signal between said terminal and said cathode, the characteristic of said tube and the values of said resistors being such that the voltage 2; at said anode is given by the equation v=log E, and means for difierentiating the voltage 1: to obtain the voltage defined by the equation i Ll dt E dt 10. In combination, apparatus installed in an aircraft for determining the time required for said aircraft to reach a target that it is approaching at constant speed, said apparatus comprising means for continuously radiating a radio wave toward said target whereby said wave is reflected back to said aircraft with a field strength E at said aircraft, means for receiving said reflected wave, means for producing from said received wave a time-from-target voltage having a value equal to dE l E and which is a measure of the time required for said aircraft to reach said target, and signal smoothing apparatus comprising a potentiometer resistor having a variable tap thereon from which a control voltage may be taken, a motor coupled to said tap to move it along said potentiometer resistor, said resistor having the characteristic that its resistance where c is a constant and where .r is the distance of the tap from its final position when said aircraft reaches said target, avacuum tube having a cathode, at least one control grid and an anode, a power supply connected to said motor through the cathode-anode impedance of said tube, a filter comprising a series resistor and a shunt capacitor and having a time constant thatis longer than the received signal fluctuations that are to be smoothed out, means for applying said timefrom-target voltage to a control grid of said tube by Way of said filter, the capacitor of said filter being connected between said grid and said cathode, means comprising a connection from said potentiometer tap to an electrode of said tube for changing the cathode-anode impedance of said tube in a sense opposite to the change in said impedance roduced by the voltage from said filter, and a relay circuit connected to said potentiometer tap for operating in response to a predetermined voltage on said tap.

11. In combination, apparatus installed in an aircraft for determining the time required for said aircraft to reach a target that it is approachceived wave a time-from-target voltage having avalue that is a measure of the time required for said aircraft to reach said target, and signal smoothing apparatus comprising a potentiometer resistor having a variable tap thereon from which a control voltage may be taken, a motor coupled to said tap to move it along said'potentiometer resistor, a vacuum tube having a cathode, a con" trol grid and an anode, a power supply connected to said motor through the cathode-anode impedance of said tube, a filter comprising a series resistor and a shunt capacitor and having a time constant that is longer than the received signal fluctuations that are to be smoothed out, means for applying said time-from-target voltage to said control grid by way of said filter, the capacitor of said filter being connected between said grid and said cathode, said cathode being connected through a cathode resistor to ground, a second vacuum tube having a cathode connected to the cathode of said first tube and having a control grid connected to said potentiometer tap and also having an anode connected directly to a posi: tive voltage source, and a relay circuit connected to said potentiometer tap for operating in response to a predetermined voltage on said tap.

EUGENE O. KEIZER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

